Transdermal patches have long been used for the administration of small molecule lipophilic drugs that can be readily absorbed through the skin. This non-invasive delivery route is advantageous for the administration of many drugs incompatible with oral delivery, as it allows for direct absorption of the drug into the systemic circulation, by-passing both the digestive and hepatic portal systems which can dramatically reduce the bioavailability of many drugs. Transdermal delivery also overcomes many of the challenges associated with subcutaneous injection by greatly reducing patient discomfort, needle anxiety, risk of accidental injury to the administrator and issues surrounding sharps disposal.
Despite these many advantages, transdermal delivery of drugs is confined to classes of molecules compatible with absorption through the skin. Delivery of small molecule salts and therapeutic proteins are not typically viable with traditional transdermal delivery, as the skin provides an effective protective barrier to these molecules even in the presence of absorption-enhancing excipients.
Microneedle (including microblade) drug delivery devices have been proposed based on a wide variety of designs and materials. Some are solid, e.g., with drug coated thereon, and others are hollow, e.g., with drug delivered from a reservoir. Some are made of metal, whereas others are etched from silicon material, and still others are made of plastics such as polycarbonate.
The number, size, shape, and arrangement of the microneedles also vary considerably. Some have a single needle, while others, especially solid microneedles, have hundreds of needles per array. Most range in size from 100 microns to 2 mm.
Microneedles have shown promise for delivery drugs intradermally and transdermally, particularly where a relatively small quantity of drug is needed such as in the case of vaccines or potent drugs.
One of the desired benefits of microneedles is of course to replace, where appropriate, conventional hypodermic needles, which can cause anxiety and/or pain for many patients. There are also benefits to delivering some drugs, e.g., vaccines, into the skin rather than via intramuscular injection. However, microneedle delivery systems often have been seen as providing quite low rates of delivery, thus limiting the usefulness of such systems by requiring either small quantities of drug formulation to be used or long delivery times. For example, typical intradermal infusion using microneedles has been documented with slow infusion rates of less than 30 mcL/hour, and low infusion volumes less than 200 mcL.